The end turns of the rotor winding in an electrical generator are restrained against centrifugal force by onepiece retaining rings located at either end of the rotor. Retaining rings are the most highly stressed component in a generator, and are of particular structural concern. The design criteria requires a high strength, high toughness material with a good resistance to stress corrosion cracking and hydrogen embrittlement. In addition, the retaining rings should be non-magnetic for optimum electromagnetic performance of the generator. A non-magnetic alloy is an alloy that does not attract magnetic substances. Retaining rings made of magnetic material have eddy currents induced in them because they are in close contact with the magnetic flux generated by conductors carrying large currents. Since magnetic materials have a higher permeability to the magnetic flux the induced eddy currents have a higher voltage than would be experienced in a non-magnetic material. These eddy currents not only represent wasted energy in the system, but their high voltage also generates heat in the rings causing undesirable operating conditions. If the rings are nonmagnetic, the energy loss is negligible and the rings remain cool.
The two alloys currently used in retaining rings for large generators are iron based austenitic alloys containing manganese and chromium. One alloy contains 18% manganese and 5% chromium while the other more recently developed alloy is more corrosion resistant and contains 18% manganese and 18% chromium. All compositions shown herein are in weight percent. Progressively higher yield strengths up to 175 ksi have been achieved in retaining rings made from both of these alloys through extensive cold expansion and nitrogen addition. As used herein yield strength is the 0.2% yield strength. The 0.2% yield strength is the stress required to produce a plastic strain of 0.2% in a tensile specimen that is tested according to a method equivalent with ASTM specification E8 ("Standard Methods of Tension Testing of Metallic Materials", Annual Book of ASTM Standards, Vol. 03.01, pp. 130-150, 1984.) The term ksi stands for kips per square inch or the unit of stress representing 1,000 pounds per square inch. Cold expansion of forged rings is a demanding manufacturing process that significantly adds to the difficulty of producing retaining rings. Fracture toughness, a measure of resistance to extension of a crack is detrimentally affected by cold expansion. Fracture toughness is measured in units of ksi times square root inch and is determined by testing in accordance with ASTM test specification E399, "Standard Test Method for Plane-Strain Fracture Toughness of Metallic Materials", Annual Book of ASTM Standards, Vol. 03.01, pp. 519-554, 1984.
It is believed 175 ksi is the upper limit of attainable yield strength in the manganese-chrome alloys referred to above. However, at the 175 ksi yield strength level the 18% manganese - 5% chromium alloy has shown a strong susceptibility to aqueous stress corrosion cracking in laboratory tests (Speidel, M.O. "Stress Corrosion Cracking in Fe-Mn-Cr Alloys", Corrosion, Vol. 32, No.5, May 1976, pp. 187-199) Several corrosion related service failures of retaining rings have also been reported by Viswanathan, R., "Materials for Generator Retaining Rings" Journal of Engineering Materials and Technology, vol. 103, pp. 267-275, Oct. 1981. The 18% manganese - 18% chromium alloy has shown a tendency towards a significant drop in yield strength with a modest increase in temperature of about 100.degree. C. A significant drop in yield strength could cause distortion or failure of the ring during operation where such modest increases in temperature can be expected. Because the 175 ksi yield strength level is utilized in current generator designs, further increases in generator size or winding density will require development of modified or new higher strength alloys.
A recently developed experimental non-magnetic alloy without manganese additions but containing 3% tantalum, designated alloy T, has the nominal composition by weight percent: 34.5% nickel, 5% chromium, 3% titanium, 3% tantalum, 1% molybdenum, 0.5 aluminum, 0.3% vanadium, 0.01% boron, and balance iron. In "Research Toward New Alloys for Generator Retaining Rings", Special Technical Publication 7922, American Society for Testing and Materials, 1982, pp. 79-103, Morris, J., and Chang, K.M., indicated that this alloy has the potential to reach properties unattainable with the manganese-chromium alloys currently used. Attainable property objectives were identified as a 200 ksi minimum yield strength and a 100 ksi times square root inch fracture toughness. Furthermore, these property objectives were obtained by processing alloy T without the demanding cold expansion processing. Hot working followed by age annealing was the only processing required.
In another recently developed alloy similar to alloy T niobium was substituted for part of the tantalum. This was done in alloy TTL described in, Ganesh, S., Viswanathan, R., "A New High Strength Material for Turbine Generator Applications", Joint A.S.M., E.P.R.I. International Conference, Advances in Material Technology for Fossil Sciences, Chicago, Sept. 1987. In alloy TTL the tantalum was reduced to 1% and 1% niobium was added. However, with this change in composition the processing step of cold expansion, sometimes hereafter referred to as cold working, had to be added to meet the 200 ksi yield strength objective. Alloy TTL could not be sufficiently strengthened by hot working and age annealing alone. Therefore additional strengthening was induced through strain hardening. Strain hardening occurs when a workpiece is permanently deformed at temperatures that are low enough to allow the strain caused by deformation to be retained in the workpiece. It is preferable to induce strain during hot working since it is much more difficult to induce strain when the workpiece is at lower temperatures and the yield strength is higher.
However, alloy TTL is restricted in the temperatures it can be hot worked. Between about 750.degree. to 850.degree. C. alloy TTL experiences a reduction in ductility that causes cracking and tearing of the workpiece during hot working. Such ductility dips are expected in niobium or titanium strengthened alloys. As a result alloy TTL must be hot worked above 850.degree. C. When hot worked above 850.degree. C. insufficient hot working strains are retained in the workpiece to meet the 200 ksi yield strength objective. Additional strain hardening had to b induced with a 25% cold working strain to meet the yield strength objective. Although the mechanical property goals were met this alloy still contains tantalum, and requires a significant amount of cold working to achieve the yield strength objective of 200 ksi. Whereas it is a primary object of this invention to provide a new nonmagnetic alloy capable of meeting or exceeding the 200 ksi yield strength objective without adding tantalum to the alloy and without inducing cold working strains in a workpiece made from the alloy.
A further object is to provide a new non-magnetic alloy having a fracture toughness of about 100 ksi times square root inch when the alloy is processed to produce a 200 ksi yield strength.